1. Field of the Invention
The present invention generally relates to a refrigerant compressor and, more particularly, to a slant plate type compressor, such as a wobble plate type compressor, with a variable displacement mechanism suitable for use in an automotive air conditioning system.
2. Description of the Prior Art
A wobble plate type compressor with a variable displacement mechanism suitable for use in an automotive air conditioning system is disclosed in U.S. Pat. No. 4,960,366 to Higuchi, the disclosure of which is hereby incorporated by reference. As disclosed therein, the compression ratio of the compressor may be controlled by changing the slant angle of the inclined surface of the wobble plate. The slant angle of the inclined surface of the wobble plate and the slant plate on which it is disposed changes in response to a change in the crank chamber pressure relative to the suction chamber pressure. Changes in the crank chamber pressure are generated by a valve control mechanism which controls communication between the suction chamber and the crank chamber.
The relevant part of the above-mentioned wobble plate type compressor is shown in FIGS. 1-3. Drive shaft 260 includes inner end portion 260a and intermediate portion 260b. Inner end portion 260a is rotatably supported by cylinder block 21 through bearing 31. The diameter of inner end portion 260a is smaller than the diameter of intermediate portion 260b. Tapered ridge portion 260c is formed at the boundary between inner end portion 260a and intermediate portion 260b of integrally formed drive shaft 260.
Slant plate 50 includes opening 53 through which drive shaft 260 is disposed. Opening 53 of slant plate 50 has a configuration as disclosed in U.S. Pat. No. 4,846,049 to Terauchi, the disclosure of which is hereby incorporated by reference. Wobble plate 60 is nutatably mounted on hub 501 of slant plate 50 such that slant plate 50 rotates with respect to wobble plate 60. Balance weight ring 80 which has a substantial mass is disposed on a nose of hub 501 of slant plate 50 in order to balance slant plate 50 under dynamic operating conditions. Annular groove 502 is formed at an outer peripheral surface of the nose of hub 501. Balance weight ring 80 is held in place by means of retaining ring 81 which is firmly fixed in annular groove 502.
Snap ring 330 is attached to inner end portion 260a, and is adjacent to intermediate portion 260b. Bias spring 340 is mounted on intermediate portion 260b, at a position between slant plate 50 and snap ring 330. One end (to the right in FIG. 1) of bias spring 340 is disposed about inner end portion 260a, adjacent to tapered ridge portion 260c. The inner diameter of the right end of bias spring 340 is smaller than the diameter of intermediate portion 260b. This right end of bias spring 340 is contained or sandwiched between tapered ridge portion 260c and snap ring 330. Accordingly, axial movement of bias spring 340 along drive shaft 260 is prevented.
Annular depression 503 is formed at a rearward (to the right in FIG. 1), radially inner peripheral region of hub 501 of slant plate 50 so as to be able to receive bias spring 340 therewithin. Pillared hollow portion 504, which has a crescent-shaped lateral cross section, is formed at a rear (to the right in FIG. 1) end surface of one peripheral region of hub 501 of slant plate 50. An axis of pillared hollow portion 504 diagonally intersects with an axis of annular depression 503 so that the rear end surface of one peripheral region of hub 501 of slant plate 50 is archedly cut out as shown in FIG. 2.
The non-tensioned length of bias spring 340 when no force acts thereon is selected such that the non-secured end of bias spring 340 does not contact any portion of the bottom surface of annular depression 503, so long as the slant angle of slant plate 50 is in a range between the maximum slant angle and a selected intermediate slant angle. However, slant plate 50 is urged towards the maximum slant angle by the restoring force of bias spring 340 if the slant angle of slant plate 50 decreases below the selected intermediate slant angle due to contact of the slant plate with the spring. When the slant angle of slant plate 50 is at a maximum, the compressor operates with maximum displacement.
In operation, when the compressor is started, impact forces which act on the internal component parts of the compressor are generated. The magnitude of the impact forces is proportional to the slant angle of slant plate 50. Since slant plate 50 will very likely stay at or close to the selected intermediate slant angle when the compressor is stopped, the intermediate slant angle is selected to be a small percentage of the maximum slant angle, that is, the non-tensioned length of bias spring 340 is selected to be small in order to reduce the magnitude of the impact forces which are generated when the compressor is restarted.
However, the vacant space between the drive shaft and annular depression 503 in which bias spring 340 is disposed, around intermediate portion 260b, is limited to a small region because the diameter of intermediate portion 260b of drive shaft 260 is large. Therefore, the diameter of the body of bias spring 340 is limited to a small value and, thus, the modulus of elasticity of bias spring 340 is limited to a small value because the diameter of the body of bias spring 340 raised to the fourth power is proportional to the modulus of elasticity of bias spring 340. Accordingly, if the slant angle of slant plate 50 decreases below the selected intermediate slant angle, the restoring force of bias spring 340 may not sufficiently urge slant plate 50 back towards the maximum slant angle.
Furthermore, pillared hollow portion 504 prevents bias spring 340 from interfering with hub 501 of slant plate 50 during the inclining motion of slant plate 50. However, the provision of pillared hollow portion 504 decreases the mechanical strength of hub 501 because the thickness of hub 501 is decreased in the one peripheral region where the hollow portion 504 is located.